In recent years, image forming apparatuses, such as printers, facsimiles, etc., employing a thermographic image forming system have been widely used. They are free of noise during operation, low in cost, provide easy down-sizing and light weight, have excellent operability and are easy to maintain.
Generally, in a thermographic image forming system, a thermoplastic image forming material having a thermoplastic ink layer (hereinafter referred to as an ink ribbon) provided on a sheet shape carrier melts the ink layer by selectively heating this ink ribbon with a thermal head, etc., transfers the melted ink on a recording medium and records an image corresponding to the heated shape.
Conventional ink ribbons have a problem that print quality drops for a recording medium with a surface that is not smooth. To achieve a satisfactory printing on a recording medium having an uneven surface, an image forming apparatus uses an ink ribbon with thermal expansible particles dispersed in a heat soluble ink layer or in a separately provided ink layer (hereinafter referred to as a micro capsule).
On this image forming apparatus, when the ink ribbon is heated by a thermal head, the ink layer is melted and micro capsules expand. The ink is attached to a recording medium by the expansion pressure of the micro capsules. Thus, a satisfactory printing is performed even on a recording medium having an uneven surface.
However, there were still such unsolved problems as shown below, even with the image forming apparatus described above.
(1) The placing pressure is high. As platen pressure was set at a high level form the viewpoint of thermoplastic image forming efficiency between the ink ribbon and the thermal head, such problems were caused as the revolving load of a drum, which transports a recording medium by winding it round the drum. Down-sizing of a driving motor is difficult, due to jitter resulting from a sliding load, wear of the thermal head, etc. PA1 (2) High rate of ink ribbon consumption. Especially in color printing, ink ribbons in multiple colors are consumed, and turning costs are remarkably high. Furthermore, as a long ink ribbon is used because of large consumption, the take-up diameter of an ink ribbon becomes large.